In Drosophila, the homeobox gene tinman is absolutely required for development of the heart. In vertebrates, a small family of homeobox genes that are related to tinman have been identified, including Nkx2-5, Nkx2-3, Nkx2-6, Nkx2-7 and Nkx2-8. Although these genes are expressed in developing heart tissues, their precise role in regulation of cardiac development remains unclear. This is illustrated by experiments investigating the role of the Nkx2-5 gene in cardiogenesis. Although ablation of the Nkx2-5 gene in mouse results in embryonic death due to heart defects, the embryos contain a differentiated, superficially normal heart and the major observable defect is a failure of looping morphogenesis. The role of Nkx2-5 therefore, is not obviously similar to that of the tinman gene in the fly. This relatively minor consequences of Nkx2-5 ablation, could be explained by the presence of additional tinman family genes with redundant activities, but at present the characterization of candidate rescuing genes is very preliminary. Therefore, the importance of the tinman gene family for vertebrate heart development is currently an unresolved question. In order to further explore the role of the tinman genes during heart development, the investigators have carried out a series of experiments in the frog, Xenopus laevis. First, the investigators have identified a novel member of the tinman family that is expressed early during heart development. Second, the investigators have carried out experiments in which expression of a dominant inhibitory mutant form of Nkx2-5 in the frog embryo results in total elimination of detectable myocardial differentiation. The investigators propose that this effect is achieved by inhibition of the function of multiple members of the tinman gene family. The long term objective of the proposed research is to understand the mechanism by which the vertebrate tinman-related genes regulate cardiac development. The specific experimental aims are as follows:-1) to further examine the mechanism leading to the elimination of myocardial gene expression. 2) to use a hormone-inducible expression system to determine the time during development that Nkx2-5 plays its role in regulation of myocardial differentiation. 3). To determine whether Nkx2-5, in combination with GATA-4 and SRF is sufficient to direct myocardial gene expression. 4) To identify the complete repertoire of tinman family members expressed during myocardial development in Xenopus. Overall this research will further our understanding of the molecular mechanisms underlying formation of the vertebrate heart. This information will improve our knowledge of the causes of congenital heart defects and thereby aid in devising strategies for prevention and cure.